Abstract

We have studied the coverage-dependent chemisorption of H2 on neutral cobalt dimers and found at θ < 0.4 the chemisorption is dissociative without a precursor-mediated physisorbed state and at 0.4 < θ < 1 it is both molecular and dissociative with a ratio of 6:1. During H2 chemisorption which is entirely side-on there is a very large quenching of the magnetic moment Δμ = 4.9 μB and a linear weakening of the metal-metal bond strength, given by calculated oscillator frequencies. An approximate equation is derived that can be implemented in kinetic models to take account of the coverage-dependent decrease of the adsorption energy of H2 on cobalt clusters. We show that sub-nanometer cobalt clusters when exposed to H2 have strong coverage-dependent properties useful for the development of very sensitive H2-trace gas sensors in the sub-ppm range.